Dwarfism, which is caused by artificial modification of plant shape (particularly, suppression of elongation growth), is a considerably important target for breeding of crops. Dwarfism is an abnormality of growth which is caused as a result of a mutation in a gene involved in the regulation of normal elongation growth. The elongation growth of plants is achieved by repetition of cell division and cell elongation, which are regulated by complex influences due to various factors, such as external environment factors (e.g., temperature, light, and the like), and internal environment factors (e.g., plant hormones, and the like). Therefore, it is inferred that a variety of genes are involved in dwarfism, such as genes directly involved in synthesis or reception of plant hormones, genes involved in expression regulation thereof, and the like. Among other things, an abnormality in the biosynthesis pathway of gibberellin is well known as a cause of dwarfism.
Gibberellin biosynthesis in higher plants includes three stages, which are catalyzed by enzymes existing in plastids, endoplasmic reticular membranes, and cytoplasm, respectively. In the third stage, in which GA12 aldehyde and subsequent substances are processed, there are two pathways of early-13 hydroxylation hydroxylase and a non-hydroxylase. In both pathways, gibberellin is physiologically activated by its carbon at position 3 being hydroxylated by 3β hydroxylase, and subsequently, is inactivated by its carbon at position 2 being hydroxylated by 2β hydroxylase.
In rice, it is known that at least two 3β hydroxylase genes exist (OsGA3ox1 and OsGA3ox2) in rice. One of them, OsGA3ox2, was demonstrated to correspond to the D18 (dy) gene locus which has been long known as a rice dwarfism gene. A 2β hydroxylase gene of rice, OsGA2ox1, was isolated as a novel gene belonging to a group of 2-oxoglutaric acid-dependent enzyme genes. Attempts have been made to suppress the elongation growth of plants by regulating these enzymes directly involved in conversion of active and inactive gibberellins (Kagaku-to-Seibutsu [Chemistry and Biology], Vol. 38, 2000, pp. 131–139). Among them, an attempt was made in which by regulating the expression level of a 2β hydroxylase gene in transformed plants, the content of active endogenous gibberellin is changed so as to obtain plants having a desired height. However, when OsGA2ox1 is forcedly expressed throughout a whole plant using an actin promoter which is a conventional constitutive promoter, gibberellin is unavoidably metabolized in reproductive organs as well as stems and leaves. As a result, the content of endogenous gibberellin is radically reduced to an extent that shows superdwarfism. In such a transformant, reproductive organs which require a large amount of gibberellin are also prevented from normally growing. Therefore, when a transformed plant having dwarfism is developed by overexpression of the 2β hydroxylase gene, it is necessary to suppress the influence on reproductive organs by utilizing a promoter working specifically in vegetative growth tissue.
For tobacco, the localization of plant expression tissue of a gene encoding 3β hydroxylase (Nty gene) has been investigated (see Plant Journal (1999) 20(1), 15–24). In this research, the promoter region of the Nty gene was linked to the GUS gene, and how the gene was expressed in various tissues in plants was observed. As a result, it was found that expression of the GUS gene linked to the promoter region of the Nty gene was limited to the site of action of gibberellin, including vegetative growth tissue. This research only found a relationship between the expression of the gene encoding 3β hydroxylase and the action of gibberellin. Therefore, no vegetative tissue specific promoter, which can be practically used, has been discovered.
Therefore, if a promoter for expression in vegetative growth tissue, whose activity is high and which can be practically used, is obtained from rice genes, it can contribute much to breeding of useful plants including rice and the like.